A frame generally has two rims, each presenting an inside bezel forming a groove.
Shaping an ophthalmic lens for the purpose of mounting it in a corresponding rim of the selected eyeglass frame then consists in modifying the outline of the lens so that it matches the shape of the rim.
This shaping comprises two operations, namely edging to shape the periphery of the lens, and beveling to put an engagement ridge into place on the periphery of the lens for engaging in the bezel of the corresponding rim of the eyeglass frame.
The edging operation consists in eliminating the superfluous peripheral portion of the ophthalmic lens in question so as to bring its outline, which is usually initially circular, down to an outline of a shape that is close to the shape of the outline of the corresponding rim of the eyeglass frame.
The beveling operation consists in forming an engagement ridge, also known as a “bevel”, along the edge face of the ophthalmic lens. This engagement ridge is for engaging in the bezel formed along the inside face of the corresponding rim of the eyeglass frame, in order to hold the ophthalmic lens rigidly in the rim.
Conventionally, these operations of edging and beveling are performed by a grinding appliance that is referred to as a grinder and that possesses means for blocking the lens and for driving it in rotation, together with an edging grindwheel and a beveling grindwheel mounted to rotate for the purpose of machining said lens. The beveling grindwheel generally presents a shape that is generally cylindrical, with a beveling groove formed halfway along serving to generate the bevel on the edge face of the lens.
At the end of the beveling operation, with certain lenses, in particular lenses that are strongly cambered and lenses that are edged to have an outline presenting a zone that is substantially rectilinear, a phenomenon is observed whereby the bevel becomes thinner. This bevel-thinning phenomenon degrades the quality and the accuracy with which the lens is mounted in its frame.
This phenomenon, referred to as “paring away” of the bevel, can be explained as follows. The beveling grindwheel presents a large radius. As a result, during the beveling operation, the arc of the beveling grindwheel that is engaged in the material of the lens is extensive. Consequently, when the beveling grindwheel is machining the edge face of the lens at a given cross-section of said lens, it also machines, in involuntarily manner, a portion of the edge face of the lens that is situated in front of said cross-section and another portion of the edge face of the lens that is situated behind said cross-section. A first interference is thus observed between the beveling grindwheel and the portion of the bevel that has already been made and a second interference is observed between the beveling grindwheel and the portion of the bevel that remains to be made. These interferences thus generate this phenomenon of the bevel being thinned.
In order to minimize this problem, a solution is put forward in document FR 2 893 524. That solution consists in modifying a conventional grinder that includes a beveling grinder disk mounted to rotate about a first axis, so as to provide said grinder disk with an additional degree of freedom in movement. The grinder disk is more precisely mounted to pivot about a second axis that is orthogonal to the first axis, thereby enabling it to be oriented in such a manner that it follows the curvature of the edge face of the lens so as to reduce the extent to which the bevel is pared away. Nevertheless, such a grinder is particularly expensive to fabricate.